Integrally stiffened structures such as hat shaped stiffeners and skin structures often require the hat shaped stiffeners to have chamfered flanges to reduce stress concentration at the interface with the skin being reinforced. Undercutting of the plies positioned in the flanges is employed to achieve a chamfered configuration and a configuration to enclose ply edges of the flanges. The enclosing of the ply edges encloses carbon fibers that are exposed at the edge of the plies positioned in the flanges. Fibers such as those constructed of carbon can experience a static charge buildup at the ply edges which then requires the need for sealing, bonding and grounding of the fiber ends so as to allow discharge of the static charge without creating unnecessary electrical transmission risk, particularly in the vicinity of fuel tanks.
Chamfering un-cured composite stiffener flanges can be difficult to achieve in practice and particularly with respect to those that have shallow angles. This process can introduce significant recurring or non-recurring costs through specialized manufacturing processes and/or introduce limitations on the allowable geometry of the stiffener such as e.g. chamfer angle, pre-form thickness, curvature, pad-ups etc. Chamfering un-cured composite flanges of hat stiffeners is typically more difficult to achieve with dry materials compared with pre-impregnated materials and can introduce additional limitations on the allowable geometry such as minimum chamfer angle.
Constructing the flanges to be square and not chamfered is not structurally efficient and may not be compatible with co-curing as a single integrated hat stiffener and skin construction. The square flange edge is also a poor design for facilitating electrical static buildup at the fiber ends that would be exposed on the square edge and would require sealing after fabrication.
Staggering lay-up of individual plies, by way of manual or automated processes, of stiffener pre-forms using staggered ply drops can be used to create undercut chamfered flanges, however, application of these processes typically introduce either significant recurring and/or non-recurring costs. With using a manual process of ply by ply hand lay-up a high recurring cost is incurred. With utilizing an automated process which employs net trim limitation machines, there is a high non-recurring equipment cost which can also include limitations on achievable curvatures, ply ramps and other configurations for subsequent stiffener forming.
Use of post-form trimming for undercutting the chamfer of pre-preg stiffeners has been demonstrated in production for very thin flanges where only a small number of plies are used in the layup. Replication of this process for dry reinforcements has not yet been demonstrated and is likely to be difficult for shallow chamfer angles due to a risk of fraying the edges of the dry fiber plies.
There is a need for a method to provide chamfer undercut configurations for pre-form dry fibers in the manufacture of hat stiffeners used for stiffening reinforcement with respect to a skin structure. The method needs to provide a low cost to the fabrication of chamfered undercut flanges and be versatile to accommodate various needed thicknesses of flanges and various chamfer angles.